The invention relates to a method for recovering heat from a combustion engine, in particular of a motor vehicle, and for converting the recovered heat into mechanical energy by means of an expansion machine, wherein a heat accumulator fluid which is guided in a primary circuit by means of at least one primary pump is heated by the waste heat of the combustion engine by means of at least one first heat exchanger, is transferred into a heat accumulator and recirculated to the at least one first heat exchanger, and wherein the heat accumulator fluid is furthermore guided in a secondary circuit by said heat accumulator fluid being extracted in the vapor state from the heat accumulator and being supplied to the expansion machine, and being condensed by means of a condenser downstream of the expansion machine and being recirculated into the heat accumulator by means of at least one secondary pump. The invention furthermore relates to a device, which is suitable in particular for carrying out such a method, for recovering heat from a combustion engine, in particular of a motor vehicle, and for converting the recovered heat into mechanical energy by means of an expansion machine, with a primary circuit of a heat accumulator fluid, which primary circuit comprises at least one first heat exchanger which is heated by the waste heat of a waste heat flow from the combustion engine, a heat accumulator and a first annular line, connecting the first heat exchanger to the heat accumulator, with at least one primary pump, and with a secondary circuit of the heat accumulator fluid, which secondary circuit is connected to the primary circuit and comprises the expansion machine, a condenser arranged downstream thereof and a second annular line with at least one secondary pump.
Methods for recovering the waste heat from combustion engines in the manner of the Clausius-Rankine cycle are substantially known. They are used, for example, in large ship engines and stationary engines in order to improve the economy and the environmental friendliness thereof through the energy saving achieved therewith. Furthermore, the use of cycles of this type in lorry engines has been tested, wherein a fuel saving of up to approximately 15% was able to be obtained (Rainer Lutz, Peter Geskes, Eberhard Pantow, Jochen Eitel: “Nutzung der Abgasenergie von Nutzfahrzeugen mit dem Rankine-Prozess [Use of the Exhaust Gas Energy from Commercial Vehicles using the Rankine Cycle]”, MTZ October 2012). However, firstly the high technical and structural outlay with regard to the evaporator and condenser used for the heat accumulator fluid in the cycle and, secondly, the complicated control and regulating technology have proven disadvantageous here, and therefore this technology has been unable to be realized in practice to date. Furthermore, the technology known with regard to lorry engines is transferable at best to a limited extent to engines of passenger vehicles because, in contrast to lorries, passenger vehicles are generally operated with very different and frequently changing engine powers, and therefore the useable waste heat from the internal combustion engine greatly varies, wherein such unsteady heat flows are scarcely controllable by regulation technology for realizing the abovementioned Rankine cycles according to the prior art. Also, the achievable efficiencies are very low in the case of a forced continuous operation of such a Rankine cycle. In the meantime, there is a requirement both for economic and for ecological reasons to ensure a saving on fuel even in the case of passenger vehicles (for example in town traffic where there is maximum fuel consumption because of the frequent load changes) by using the waste heat from the internal combustion engine (Dr. Raymond Freymann: “Der Turbostreamer der zweiten Generation [The Second Generation Turbostream]”, MTZ February 2012).
DE 10 2011 105 709 A1 describes a method of the type in question and a device for recovering heat from an internal combustion engine of a motor vehicle, and for converting the recovered heat into mechanical energy by means of an expansion machine, by a heat accumulator fluid which is guided in a working or secondary circuit being evaporated by means of a heat exchanger, which is fed from the waste heat of the engine and is in the form of an evaporator, and the vapor which is produced is supplied to an expansion machine which is coupled to the engine and converts the thermal energy into mechanical energy. The exhaust steam is condensed by means of a condenser downstream of the expansion machine, is evaporated by means of the evaporator and supplied again to the expansion machine. In order to ensure improved efficiency of the drive system even in the event of an unsteady operation of the internal combustion engine with varying waste heat flows, the document makes provision for the working or secondary circuit to be connected via a multi-way valve to a primary circuit of the heat accumulator fluid, which primary circuit comprises a steam accumulator for the heat accumulator fluid evaporated by means of the evaporator, and therefore, in the event of a driving power of the expansion machine temporarily not being required, the heat accumulator fluid in the vapor state can firstly be temporarily stored and secondly can be extracted as required and output to the expansion machine if an increased power is intended to be provided.
A disadvantage firstly consists in that the thermal energy which can be stored by means of the steam accumulator has proven relatively small if the steam accumulator does not have a very large capacity which takes up the available loading compartment of the motor vehicle and, in addition, also causes a considerable additional weight in respect of the necessary thermal insulation of the steam accumulator, said additional weight resulting in turn in increased fuel consumption. Consequently, the steam stored in the steam accumulator is firstly capable of bridging only short load phases of the engine when said steam is supplied to the expansion machine and secondly, in phases of a theoretical excess of available waste heat from the internal combustion engine, only a small part thereof can be stored in the steam accumulator. Furthermore, the integration, which is proposed in DE 10 2011 105 709 A1, of the evaporator in the working or secondary circuit (i.e. the heat accumulator fluid evaporated by means of the evaporator is supplied directly to the expansion machine during normal operation) requires in particular a considerable outlay on control and regulating technology since the quantity of heat accumulator fluid to be evaporated always has to be adapted to the specifically available waste heat flow from the combustion engine. Furthermore, the control and regulating technology has to take account here of a multiplicity of operating states, in dependence on which the primary and secondary circuits of the heat accumulator fluid have to be diverted in various ways by means of the multi-way valve.
JP H03-018656 A concerns a further method for recovering the heat from internal combustion engines of motor vehicles, wherein the recovered heat is converted into mechanical energy by means of an expansion machine. In this case, a heat accumulator fluid which is circulated by means of a pump is heated by the waste heat of the internal combustion engine by means of a heat exchanger of the cooling water circuit, is transferred into a type of heat accumulator and recirculated to the heat exchanger of the cooling water circuit. Furthermore, it is provided that a partial flow of the heat accumulator fluid is guided in a branch circuit by said partial flow being extracted in the vapor state from the heat accumulator and being supplied to a low load stage of the expansion machine, being condensed by means of a condenser downstream of the expansion machine and being recirculated into the heat accumulator by means of a further pump. Alternatively, the heat accumulator fluid is extracted in the liquid phase from the heat accumulator, is evaporated by means of an exhaust gas heat exchanger and supplied directly to a high power stage of the expansion machine.
WO 2012/074456 A1 likewise describes a method and a device for recovering heat from internal combustion engines of motor vehicles and for converting the recovered heat into mechanical energy by means of an expansion machine, wherein a heat accumulator medium in the form of a zeotropic coolant mixture is guided in a plurality of circuits with a separator. In a first circuit, the coolant mixture is evaporated in an evaporator, which is operated with waste heat from the engine, and is transferred into the separator from where the vapor phase of the low-boiling coolant component is supplied to the expansion machine via an additional heater and is recirculated via a heat exchanger and a condenser. In a second circuit, the liquid phase of the high-boiling coolant component is extracted from the separator and—in turn via the heat exchanger and the condenser—transferred to the evaporator from where said coolant component can pass via the additional heater to the expansion machine or back into the separator.
Apart from the fact that firstly the heat accumulator according to JP H03-018656 A cited above is capable of storing only a very small portion of the available heat (namely only the heat which is extracted from the cooling water circuit via the heat exchanger of the coolant circuit and serves merely for operating the expansion machine in the low power stage by means of the heat accumulator medium removed from the gas phase of the heat accumulator, while the exhaust gas heat exchanger is arranged directly upstream of the expansion machine), secondly the separator according to previously cited WO 2012/074456 A1—if it serves at all (also) as a “heat accumulator”—can likewise store only a very small portion of the available heat (namely exclusively for a low load state in which the gaseous phase of the low boiling coolant component is extracted from the gas chamber of the separator and supplied via a valve and the additional heater to the expansion machine whereas, in a high-load state, only cold liquid coolant is extracted from the separator via a further valve in order to heat said coolant only by means of the heat exchanger and the evaporator), in both known methods in particular the very high outlay on control and regulation which has to conform with the respective operating states in turn proves disadvantageous, wherein the heat which generally arises in a highly unsteady manner in combustion engines cannot be effectively stored for the abovementioned reasons.
The invention is therefore based on the object of developing a method and a device for recovering heat from a combustion engine, in particular of a motor vehicle, and for converting the recovered heat into mechanical energy by means of an expansion machine of the type mentioned at the beginning, in a simple and cost-effective manner to the effect that the abovementioned disadvantages can be at least partially countered.